TTYs (also known as TDDs) are text terminals that people with hearing impairments use in order to communicate over telephone lines. In the United States, the most commonly used TTY communication protocol is the technique specified by ANSI/TIA/EIA 825 (“A 45.45 Baud FSK Modem”). Relevant characteristics of this protocol include the following:
(1) TTYs are silent when not transmitting. Unlike fax machines and computer modems, TTYs have no “handshake” procedure at the start of a call, nor do they have a carrier tone during the call. Although this approach tends to limit the speed of transmission, it has the advantage of permitting TTY tones, DTMF (Dual Tone Multi-Frequency signals, also known as “touch tones”), and voice to be intermixed on the same call.
(2) Operation is “half duplex.” TTY users must take turns transmitting, and typically cannot interrupt each other. If both people try to type at the same time, their TTYs will show no text at all, or will show text that is gibberish. There is no automatic mechanism that lets TTY users know when a character they have typed correctly has been received incorrectly. To avoid this problem, a user normally types a message and then types “GA” (for go ahead). This does solve the problem of half duplex operation but results in a slow, jerky operation which tends to limit spontaneous interaction.
(3) Each TTY character consists of a sequence of seven individual tones. The first tone is always a “start tone” at 1800 Hz. This is followed by a series of five tones, at either 1400 or 1800 Hz, which specify the character. The final tone in the sequence is always a “stop tone” at 1400 Hz. The “stop tone” is a border that separates this character from the next. Each of the first six tones is 22 milliseconds in duration. The final “stop tone” may also be 22 milliseconds, but is permitted to be as long as 44 milliseconds. This means that the duration of each TTY character is at least 154 milliseconds, which works out to approximately six and a half characters per second. (The description of this as a “45.45 Baud” protocol is based on the number of 22-millisecond tones that can be transmitted in one second, not the number of characters.)
From a usability perspective, one of the benefits to using a half-duplex, carrier-free protocol for TTYs is that it is possible to intermix voice and TTY transmissions on the same call. This is important because nearly half the people who use TTYs are individuals with moderate hearing loss who nevertheless are able to speak clearly; these individuals often prefer to receive with their TTYs and then speak in response, a process commonly referred to as Voice Carry Over or VCO. Another common pattern is for individuals (including those who hear adequately but do not speak clearly) to alternate between TTY and voice on the same call, relying on voice for informal conversation and TTY for critical information such as addresses and credit card numbers.
From a usability perspective, this protocol also has several disadvantages, including: (1) TTY users must take turns typing to each other, and are unable to interrupt each other. (2) VCO users (individuals who prefer to receive via TTY and then speak in response) need a TTY device or specialized VCO telephone in order to communicate. (3) People who receive a TTY message in their voicemail mailbox (which may include mailbox owners who have no communication disabilities, and therefore no easy access to a TTY), need a TTY device or specialized software to read the message. (4) The absence of handshake tones means that there is no mechanism by which to detect that a person is a TTY user until that person starts typing. (5) The protocol itself, although very robust when used in conjunction with traditional circuit-switched analog or digital telephony systems, tends to be unreliable when used in telephony systems that employ packet switching (e.g., Voice over Internet Protocol networks) or voice-optimized audio compression techniques (e.g., the GSM encoding used in many wireless systems).
FIGS. 1 and 2 illustrate the problems of performing VCO operations with a standard, 45.45 Baud, TTY such as TTY 102. To transmit or receive TTY information to other party 109, user 108 has to place the handset 107 into TTY 102 as illustrated in FIG. 1. FIG. 3 illustrates a pictorial view of a telephone such as shown in FIG. 1 having telephone 301 and TTY 302. If user 108 wants to use VCO operations, user 108 has to receive TTY information as illustrated in FIG. 1, but speaks by first removing handset 107 from the acoustic coupler of TTY 102 as illustrated in FIG. 2 and then speaking. To once again receive TTY information, user 108 has to place handset 107 back into the acoustic coupler of TTY 102 before other party 109 transmits TTY information to TTY 102. This type of VCO operation is awkward at best.
The 45.45 Baud FSK protocol has been used in United States TTYs since 1963, and is based largely on the protocol that was used in military teletypewriters during the Second World War. Quite obviously, modern techniques would permit the development of new protocols that retain the advantages of the current protocol, while eliminating the disadvantages. Although many new protocols with excellent capabilities have been proposed, an important barrier stands in the way of their general acceptance: by some estimates, as many as 40,000,000 TTYs that use the 45.45 Baud protocol have been manufactured and distributed since 1963. This constitutes an enormous embedded base that cannot be upgraded or replaced economically. For this reason, it is desirable to solve the usability problems in a manner that does not require the 45.45 Baud protocol, and millions of current-generation TTY devices, to be abandoned.
Among the above-listed problems associated with the 45.45 Baud protocol, the issue that has been addressed explicitly in prior art concerns the inability of packet switched networks or voice-optimized compression codecs to support reliable TTY communication. Attention has been paid to these problems largely because, in the United States, Section 508 of the Workforce Investment Act of 1998, as well as Sections 251(a)(2) and 255 of the Telecommunication Act of 1996, require telephony systems to be TTY compatible.
In the prior art, it has been recognized that TTY signals need not be transmitted as audio tones, and may instead be converted and transmitted as digital information in a more reliable non-audio data channel. U.S. Pat. No. 6,351,495 discloses a cellular transmission system where TTY audio signals are recognized and converted to digital information for transmission via the signal transmission portion of the cellular system. Also, in the prior art for VoIP systems, it is known to encode precision tones such as multi-frequency dial tones as digital information and transport this digital information to a destination point via a signaling channel that is distinct from the bearer channel used for encoded voice. One such VoIP system that transports multi-frequency dial tones in this manner is disclosed in U.S. patent application Ser. No. 09/18,909, entitled “Integration of Remote Access and Service”, filed Nov. 22, 2000, and assigned to the same assignee as the present patent application. U.S. patent application Ser. No. 09/18,909 is hereby incorporated by reference. It is important to note, however, that objective of these approaches is to permit the original audio tones to be reconstructed accurately at the receiving location. Although the problem of reliable transport for TTY signals on wireless or VoIP networks is addressed by these approaches, a problem that remains is that a user who receives these transmissions still requires a TTY device or specialized software to translate the transmissions into human-readable text.